Pulse radar altimeters demonstrate superior altitude accuracy due to their inherent leading edge return signal tracking capability. The pulse radar altimeter transmits a pulse of radio frequency (RF) energy, and a return echo is received and tracked using a tracking system.
In a typical pulsed radio altimeter application, one to two radar strip antennas are mounted side by side on the underside of an aircraft, with their long axes oriented along the X-axis of the vehicle. In an unambiguous interferometric altimeter system a third antenna, referred to as the ambiguity antenna, is added. Referring to FIG. 1, an ambiguous antenna 6 serves as the transmitter, and all three antennas 4, 6, and 8 serve as receivers. The radar system tracks the closest ground returns by closing a control loop on the leading edge of a reflected radar pulse 13 received by the ambiguous antenna 6. The two outer antennas 4, 8 are used, in conjunction with the ambiguous antenna, to determine the cross-track angle of the first ground return. Cross-track angle θ is the angle between the vertical axis in the aircraft body frame and the object being detected also referred to as the mechanical angle.
The three radar antennas, 4, 6, 8, are arranged side-by-side, with the center 6 antenna closer to the left antenna 4 than to the right antenna 8. While in this non-limiting embodiment, the center antenna 6 is closer to the left antenna 4 rather than the right antenna 8, but, in fact, the only requirement is that the spacing between the antennas is not equal. These antennas 4, 6, 8 represent the baseline of a triangulation system that measures the electrical phase angle φ for the reflected radar pulse 13 from a target 11 between each pair of antennas: left antenna 4 to ambiguous antenna 6 (also referred to as “L-A”), left antenna 4 to right antenna 8 (“L-R”), and center antenna 6 to right antenna 8 (“A-R”). The relationship between the measured electrical phase angle φ and the cross-track (or ‘mechanical’) angle θ of the return, as measured from the antenna centerline, is determined by the wavelength of the radar signal and the separation of the two antennas in each pair. For any given return each pair of antennas will detect a different phase angle, due to the differing separations of the antennas.
Along with the three antennas, three processing channels, referred to below as left, right and ambiguous respectively, each include a receiver, a data acquisition device, range gate, and a filter. Use of the three antenna system, along with the processing described herein, provides a solution to the ambiguous detected angle of the nearest object. The ambiguous detected angle is due to the spacing of the antennas being greater than the transmitted RF frequency wavelength. By receiving three returns, the processing system using a phase comparison process is able to determine an unambiguous location of the nearest object on the ground.
During the phase comparison portion of the time interval, the Doppler filters of the left, right and ambiguous channels are set to select a swath (shown in the nonlimiting exemplar depicted in FIG. 1 as ranging from a −40 to +40 degrees relative to a vertical) below aircraft. A phase processor compares the phase difference between R1 and RA, R2 and RA, and R1 and R2 (as set forth in U.S. Pat. No. 6,856,279, incorporated by this reference) once the return signals are received. As illustrated in FIG. 1, the exact range differences ΦLA, ΦRA, and ΦLR are from phase differences and simple trigonometry relations are used to determine the exact crosstrack distance to the target 11 in aircraft body coordinates.
As illustrated in FIG. 1, after the range differences ΦLA, ΦRA, and ΦLR are determined and knowing the antenna separations SLA and SAR, and measured range R1, then the crosstrack distance (Y) and vertical distance (Z) can also be computed in aircraft body coordinates. It is important that the precise location of target 11 in each swath is determined so correlation can be made with the electronic maps which will accurately locate the aircraft on the electronic map. For example, at typical high speed aircraft cruising velocities, a radar, configured with reasonably sized Doppler filters, has swath widths of approximately 10 feet at 5000 feet altitude. The resulting incidence angle indicating the bearing of the target relative to a line normal to the surface of the center antenna 6 will then be on the order of less than 3 degrees. Basic trigonometry relations show that even with a typical error (for example 1%) on the radar range gate measured distance R1, (50 feet at 5000 feet altitude), knowing the precise antenna separation 50, and precise range differences ΦLA, ΦRA, and ΦLR, the crosstrack distance (Y) will be precise due to the very small incidence angle encountered.
Due to the geometry of the system it is possible for the electrical phase angle representing the phase difference between pairs of antennas to exceed 360 degrees, so that a radar return far from the antenna centerline may have the same electrical phase angle as a return that is nearer to the antenna centerline. This produces ambiguity when trying to determine mechanical angle when only electrical phase angle is known. For example, with a 1-foot separation between antennas L and R and a 4.3 GHz radar signal (wavelength=0.2291 feet), the electrical phase angle will increase from 0 to +180 degrees as the ground return moves from a mechanical angle of 0 degrees (straight below the aircraft) to an angle of 6.55 degrees. As the mechanical angle increases, the electrical phase angle will wrap back to 0 degrees at a mechanical angle of 13.1 degrees. Therefore, with a 2-antenna system it would be impossible to tell the difference between a ground return directly below the aircraft and a ground return 13.1 degrees to either side of the aircraft.
As the prior art process is currently executed, the solution of the probable location of the target requires a great deal of calculation. For each first spacing, for example ΦLA, the other two angles have to be checked requiring a calculation of ΦLR and ΦAR to check for convergence. As such, the calculation times are a function of the product of each of the possible solutions for each antenna separation. Processing time becomes extremely expensive, either in terms of performance or raw cost. The immense processing overhead the system portrayed in FIG. 1 generates is demonstrated by selection of real numbers.
In a non-limiting example depicted in FIG. 2, the L-A antenna separation is taken to be 4 inches, and the A-R antenna separation is taken to be 8 inches, then the L-R antenna separation is the sum of the distances, 12 inches. Assuming an exemplary RF frequency of 4.3 GHz, and an antenna beamwidth of 80 degrees, in order to unambiguously determine the mechanical angle of the ground return over the range −40 to +40 degrees, the 13 elements result in 75 possible combinations of electrical phase wraps, when we take one element from each of the 3 rows. The mechanical angle ranges over approximately 80 degrees. Because electrical angles are really phase measurements relative to the RF frequency, phase can wrap through a number of complete cycles such that triangulation may be based upon on angle θ′ which is equal to either the original θ or the original θ plus or minus a integral multiple of 360 degrees.
A well known relationship exists to generate the possible locations at which the electrical angle φ to the mechanical angle θ. FIG. 2 shows the generation of all possible combinations relating the mechanical angles θ to the electrical angles φ. (Numeric superscripts are used not to denote squares and cubes but rather variable names such that θ3 expresses the third possible value for θ and not the value θ raised to the third power). The combinatorial mathematics shows that seventy five possible combinations exist as to the combinations of mechanical and electrical angles resulting from the exemplary dimensions of the model shown in FIG. 1.
Processors according to the prior art perform a brute-force search of all 75 possible combinations in order to find the correct solution. First, the mechanical angle for all 13 elements of exemplary array of three antennas is computed. Then, each of the 75 possible combinations of antenna pairs (one element from each of the 3 rows in FIG. 2) is considered. The variance of the three selected mechanical angles is computed for each of the 75 combinations. The combination that has the lowest computed variance is taken to be the correct solution. Finally, the weighted average of the three computed mechanical angle values is computed. In this way the mechanical phase angle is determined from the electrical phase angles unambiguously.
What is needed in the art is a processor and a method for removing those of the solutions that are inherently nonconvergent so as to remove the inherent overhead of performing those calculations and checking their meaning against the situation in order to lighten the processing burden.